1. Field of the Invention
The present invention relates to an improved surface emitting semiconductor laser array, and more particularly relates to a matrix driving type surface emitting semiconductor laser array improved in power consumption by reducing the resistance of the current passage.
2. Description of Related Art
For using in the fields of optical exchange and optical information processing, surface emitting laser arrays having a two-dimensional matrix array of surface emitting lasers (VCSEL namely Vertical Cavity Surface Emitting Laser) are required, particularly large scale surface emitting semiconductor laser arrays having a large number of surface emitting semiconductor lasers are required.
To use a large scale surface emitting semiconductor array, it is required to drive respective component surface emitting lasers independently, the respective elements should be connected independently in the connection arrangement, therefore, in the case of an independent driving type surface emitting semiconductor laser array of M rows and N columns, M.times.N connection wiring is required.
A matrix type driving system has been developed in which a plurality of row direction lines for connecting the respective surface emitting lasers in the row direction in parallel are provided on the front surface of the surface emitting semiconductor laser array, and on the other hand a plurality of column direction lines for connecting the respective surface emitting lasers in the column direction are provided on the back surface of the surface emitting semiconductor laser array, then one of the row direction lines and one of the column direction lines are selected to select and light the surface emitting laser located at the intersection position.
In this matrix driving system, a surface emitting semiconductor laser array of M rows and N columns may have M+N electrodes.
One column of the matrix driving type surface emitting semiconductor laser array is described with reference to FIGS. 15 to 17.
FIG. 15 is a perspective view, FIG. 16 is a plain view, and FIG. 17 is a side view viewing from the right in FIG. 15 of a matrix driving type surface emitting semiconductor laser array.
In manufacturing the surface emitting semiconductor laser array, by use of molecular beam epitaxy, a bottom contact layer 52 comprising an n-type GaAs layer is formed on a semi-insulating GaAs substrate 51, and an n-side multilayer reflection film 53 with the total thickness of several .mu.m having alternately laminated AlAs layers and GaAs layers being 1/4 of in-medium wavelength in respective film thickness is formed on the bottom contact layer 52. Next, an undoped active layer 54 having the same film thickness as an in-medium wavelength and having a laminate structure comprising two Al.sub.0.4 Ga.sub.0.6 As layers with interposition of a laminate quantum well layer comprising three layers of two GaAs layers with a thickness of 10 nm and one In.sub.0.2 Ga.sub.0.8 As layer inserted between the GaAs layers is formed, and on the undoped active layer 54, a p-side multilayer reflection film 55 with the total thickness of several .mu.m having alternately laminated AlAs layers and GaAs layers being 1/4 of in-medium wavelength in respective film thickness is formed on the undoped active layer 54. In these processes, Si is used as an n-type dopant and Be is used as a p-type dopant respectively. Next, by applying reactive ion etching, grooves 56 for wiring separation are formed to provide M rows in vertical direction (top-bottom direction in FIG. 16). The depth of the grooves 56 extends through the bottom contact layer 52 to the semi-insulating GaAs substrate 51 for electrical separation of rows. The grooves 56 are filled with polyimide. Next, by applying vacuum evaporation, an Au layer is formed on the p-side multilayer reflection film 55, and the Au layer is removed from the boundary areas of the respective columns to form N p-side electrode wiring 57 extending in the column direction (right-left direction in FIG. 16). Proton is injected on areas between p-side electrode wires 57, which define the column direction lines, so as to penetrate from the p-side multilayer reflection film 55 at least to the active area 54, and the groove area is insulated to separate the rows. Only on the ends (near side end in the drawing), etching is continued until the bottom contact layer 52 comprising an n.sup.+ GaAs layer is exposed. On the exposed surface, electrode pads (n-side electrode pads) 58 for respective rows are formed. On the ends of p-side electrodes 57 which define respective column direction lines, electrode pads (p-side electrode pads) 59 for respective columns are formed.
To select an arbitrary point (i, j), row i and column j may be selected. In a surface emitting semiconductor laser array having the above-mentioned structure, because the p-side electrode wires are of gold, the resistance of the p-side electrode wires 57 is as low as several .OMEGA. and does not cause any problem, however, because the bottom contact layer 52 is served as the contact line to the n-side electrodes, the line resistance of a surface emitting semiconductor laser located far from an n-type electrode pad 58 is significantly large. Therefore, the power consumption of the matrix driving type surface emitting semiconductor laser array having the above-mentioned structure is significantly large.
The above-mentioned matter is described further in detail with reference to FIG. 15 again and FIG. 18.
In the surface emitting semiconductor laser array shown in FIG. 15, the bottom contact layer 52 consisting of n.sup.+ GaAs layer is used as the bottom side wiring, the contact resistance between an electrode and semiconductor is at largest several .OMEGA. or lower. Herein the contact resistance is confirmed analytically using Transmission Line Model method. In the example shown in FIG. 15, the contact between an electrode and a semiconductor is an electrode pad 58. FIG. 18 shows a cross sectional view of FIG. 15, and the effective contact distance between the electrode pad 58 and the bottom contact layer 52 comprising an n.sup.+ GaAs layer is operated arithmetically. The contact resistance is determined from the product of contact specific resistance and contact area where a current flows through. In this case the electrode material is metal, and the resistance and contact specific resistance are small. However, because the resistance of the n.sup.+ GaAs layer is comparatively large, the electrical effective contact area with the bottom contact layer comprising an n.sup.+ GaAs layer extends to the position shown with Lt in FIG. 18. Assuming that the contact specific resistance .rho.c is 10.sup.-6 cm.sup.2, the specific resistance rs of the n.sup.+ GaAs layer is 3.times.10.sup.-4 cm .OMEGA. (determined under the assumption that the carrier concentration n is 10.sup.19 cm.sup.-3 and carrier mobility .mu. is 5000 cm.sup.2 /V sec using 1/e n.mu.. Wherein e is an elementary charge of 6.times.10.sup.-19 coulomb), and the length of an electrode pad d (horizontal direction of the paper plane in FIG. 18) is 1 mm, Lt is calculated from the equation .rho.c=(Lt.times.(.rho.crs)).div.coth (d/Lt) and then 10 .mu.m is obtained as the result. Therefore, in the case of the electrode pad with a width of 10 .mu.m, the effective contact area is 10 .mu.m.times.10 .mu.m. The contact specific resistance of 10.sup.-6 cm.sup.2 gives the contact resistance between an electrode and semiconductor of 1 .OMEGA., this value is sufficiently smaller than the resistance of the surface emitting laser body.
However, in the surface emitting semiconductor laser array shown in FIG. 15, because the n.sup.+ GaAs layer is used for the bottom side wiring, the wiring resistance calculated based on the specific resistance of n.sup.+ GaAs.times.wiring length.times.cross section of the wiring (assuming wiring length of 1 mm, width of 20 .mu.m, and thickness of 4 .mu.m) gives 50 .OMEGA., and this value is not negligible.
As described hereinabove, in the case that the semiconductor layer is inserted in the laser structure and a current is supplied to each laser element from the end electrode pads through the contact layer, there is no problem around the electrode pads because of the small wiring resistance, but the resistance increases with distance from a pad because of high specific resistance of the semiconductor. In the case that a number of arrays are formed and the array is matrix- driven, the array is disadvantageous in that the light intensity and response speed for starting of emission depend on the location of the laser element from the vicinity of an electrode pad to the area located far from an electrode pad.
R. A. Morgan et al. discloses a structure in which the metal electrode is provided on the laser element side to reduce the wiring resistance ("Two-dimensional Laser Array display", IEEE Photonics Tech. Let., Vol. 6, No. 8 (1994) 913-917), however, it is difficult to reduce the contact resistance between a DBR layer and metal electrode because a metal electrode is formed on a conductive DBR layer (usually formed of AlGaAs material for a VCSEL which uses a GaAs substrate). The reason is that (in the example shown in FIG. 15, the shorter the wavelength is) for sufficient beam reflection on the DBR layer when a laser is oscillated and for preventing the beam from being absorbed, the more it is required that the energy gap of the DBR layer is made large by increasing the AlAs mixed crystal ratio while the increased AlAs causes the increased contact specific resistance. For example, when an Al.sub.0.3 Ga.sub.0.7 As layer, which has a relatively small contact specific resistance (contact specific resistance 9.times.10.sup.-4 cm.sup.2), is used, the contact resistance between a metal electrode and this DBR layer is approximately 100 .OMEGA.. As the result, if the wiring resistance in the metal electrode is reduced, the contact resistance increases, and the increased power consumption and poor response speed cannot be improved. On the other hand, there is another advantage that, if the AlAs mixed crystal ratio is reduced to reduce the contact resistance, the beam is absorbed in the DBR layer to fail to generate a laser beam.
In the case of a semiconductor laser array having a structure with a large difference between the peak and bottom, because the top matrix wiring electrode is inevitably formed on the area having the large surface gap, the gap between laser elements is apt to cause disconnection. Therefore, it is required that the bottom electrode is formed at the level as high as possible so that a structure with a reduced gap difference is formed. However, in the case of the conventional structure, because the top electrode and the bottom electrode intersect each other with interposition of a relatively thin insulating film, the intersection functions as a relatively large electric capacitor. As the result, the larger the number of elements of the array is, the poorer the response speed becomes due to the electric capacity, which is a problem. On the other hand, in the case of a laser array as shown in FIG. 15, there is no such problem because of no intersection of electrodes on the area other than that of laser elements.
As described in detail hereinabove, the matrix driving type surface emitting laser array is inevitably disadvantageous in that the resistance of the portion other than the surface emitting laser elements is large, and therefore the power consumption is large. It additionally causes a problem of reduction in driving speed.
Accordingly, it is the object of the present invention to provide a matrix driving type surface emitting laser array having the reduced resistance of the portion other than element bodies of the surface emitting laser which is a component of the surface emitting laser array and excellent in the response speed.